Theoretical astrophysics and plasma physics at RPC

Abstract:

Previous studies (Jordan 1975, 1980) have found that the resonance lines of He I and He II appear to have higher fluxes than expected from models made using other transition region lines formed at similar temperatures. The enhancement factor was shown to be less in coronal holes than in the quiet Sun. These results referred to observations made with low spatial resolution. Using the higher spatial resolution provided by the CDS instrument onboard SOHO, we have started a systematic re-examination of the behaviour of the He I and He II line fluxes by examining several quiet Sun regions, from Sun-centre to near the limb, including a coronal hole. While the helium line fluxes are overall relatively lower in the coronal hole, their behaviour in the quiet Sun differs between the supergranulation cell boundaries and cell interiors. New simultaneous observations with the SUMER instrument are being used to investigate possible correlations between the electron density, non-thermal velocities and the helium line fluxes. We comment in passing on tests of the accuracy of the calibration of the CDS/NIS2 spectra.

Abstract:

We analyze a kinematically unbiased sample of 5610 stars around the south celestial pole that (i) have MK spectral types in the Michigan catalogues with luminosity class V and (ii) had photometric parallaxes that placed them within 80 pc of the Sun. We bin the stars by B - V and determine for each bin the solar motion from proper motions alone. As expected, the U and W components of the derived solar motions do not vary significantly from bin to bin, while the V component varies systematically. As the classic Stromberg relation predicts, V is a linear function of the variance S-2 within each bin around the solar motion. Extrapolating V (S-2) to S = 0 we determine the solar motion with respect to the LSR, obtaining a significantly smaller value of V than is usually employed. Parenago's discontinuity in the dependence of S-2 on spectral type emerges with exceptional clarity.